The present invention relates to a development unit that develops electrostatic latent images.
FIG. 1 shows a development cartridge 128 that is used mounted in a laser printer. The development cartridge 128 is partitioned into a holding chamber 134a that holds toner and a development chamber 134a where the toner is used to develop images. The holding chamber 134a and the development chamber 134b are in fluid communication with each other through an opening 137. An agitator 155 is provided in the holding chamber 134a. The agitator 155 rotates to transport toner held in the holding chamber 134a, through the opening 137, and into the development chamber 134b. A supply roller 133, a developing roller 131, and layer thickness regulating blade 132 are disposed in the development chamber 134b. 
When the development cartridge 128 is properly mounted in the laser printer, the developing roller 131 is disposed in confrontation with a photosensitive drum of the laser printer. When the laser printer is operated in this condition, first as the agitator 155 rotates, the agitator 155 conveys toner from the holding chamber 134a to the development chamber 134b. Rotation of the supply roller 133 supplies the toner to the developing roller 131. As the developing roller 131 rotates, the layer thickness regulating blade 132 regulates thickness of toner on the surface of the developing roller 131 to a thin film of fixed thickness.
As the developing roller 131 rotates further, the thin film of toner is brought into confrontation with the photosensitive drum. At this time, the toner develops an electrostatic latent image formed on the surface of the photosensitive drum into a visible toner image. The visible toner image is then transferred onto a sheet. In this way, a desired toner image can be formed on the sheet.
If insufficient toner is supplied to the supply roller, then insufficient amount of toner per unit surface area will be carried on the developing roller. As a result, the charge per unit area of toner will be increased. This higher charge results in less toner being shifted to the photosensitive drum during development of the electrostatic latent image, so that density of a resultant visible toner image will be lowered. Consequently, the toner image on the sheet will also be thin.
Such poor print density is most noticeable in images printed before the agitator has conveyed sufficient toner to the development chamber or after the laser printer has been left unused for a fairly long time.